I was thinking of 1" RHS ali frame all welded together then ali checker plate on the outside except the top.
Then the thickness is filled with something, fire proof, such as rockwool. Or EVA if I cant be bothered.
The inner is then lined with a polcarb box and the top has a sheet of polycarb.

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Well it's not even that big.
900W x 250H x 350D from a quick measure.
This will only hold about HALF my pack.
Looking at the original weight distribution I will have to put the other half in the engine bay anyway to get it back near 50/50.

Help prevent road rage - get outta my way! Blasphemy is a swear word. Magnetic North is a south Pole.

I'll be making some battery boxes soon too.
Both these fuel tanks will be replaced with boxes that hold 12 x CALB 180 cells , and I'll need another box holding 21 cells under the bonnet .

Constructing a box & lid is one thing, but I assumed the cells all need to be clamped down somehow, this is the tricky bit. None of the many battery box images across the internet seem to have such clamping arrangments. Are such clamps required or not ??

Yeah I'm thinking that RHS with an outer plate and inner polycarb liner is over the top.
Plus it takes up a lot of space.
Also the internal gap, which I was thinking of putting rockwool etc would make the box flex with 80kg of battery pushing it around corners.

Angle and flat plate could work.
On the top I would have the angle going out for a mounting flange.

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I reckon this will work pretty well with the Leaf modules. However if you can, try and squeeze them in so there's good contact all the way out to the sides of the enclosure. Then it should be able to soak any heat away. The polycarb lining will limit how effective this is, but you will need something there for electrical isolation too.

Worst case I'd be pulling 3C for 10secs.
Mostly would be under 0.5C.
I'd be surprised if heat would be an issue in this case.
It's not a performance car - just a general runner.
You know all show - no go

The polycarb wont add much so I'll keep it thin like 1.5mm.

The BMS has a temp sensor on each cell so I'll know if it goes north so to speak...

I don't know if there are any other materials that would be better.
You know good thermal and good electrical isolator.

The cells will be packed in tight except for the tops due to the BMS, cell interconnects and power cabling to first and last cell.

I was thinking of making a wooden mock-up first just in case there is some obvious problem once mounted in the car.

Help prevent road rage - get outta my way! Blasphemy is a swear word. Magnetic North is a south Pole.